We launched the Body Weight & Puberty Study (BWPS; www.bodyweightandpuberty.niehs.nih.gov) the first pediatric-focused study ever to be conducted in the NIEHS Clinical Research Unit (CRU), in December 2015. We have enrolled over 80 subjects. In this protocol, pre-menarchal girls between the ages of 8 and 14 undergo a physical exam (Tanner staging of breast and pubic hair; height, weight, and waist-hip ratio), breast ultrasound (morphological staging of glandular development), pelvic ultrasound (ovarian and uterine volumes, follicle counts, endometrial thickness), DEXA (% body fat), hand x-ray (bone age), and blood and urine tests (sex steroids, vaginal maturation index VMI of urogenital epithelial cells) Visit 1. Study procedures are repeated in 6 months Visit 2. Findings: 1) obese girls staged as pubertal on physical exam (Tanner stage > 2) tend to have immature breast morphology on ultrasound relative to their normal weight counterparts, and 2) breast maturation scores correlate with other end-organ effects of estrogen (eg ovarian size, uterine size, bone age, and VMI) to the same degree in overweight/obese and normal weight girls, suggesting that thelarche in overweight/obese girls is driven by activation of the central components of the reproductive axis and hence ovarian estradiol, as opposed to peripheral sources of estradiol(eg adipose tissue). The accompanying manuscript is now in press at the International Journal of Obesity.